In our country, only about 30% of the primary energy supply is transformed into available energy and, accordingly, ultimately about 70% of the energy supply is discarded as heat into the atmosphere. Heat evolved by combustion in industrial plants, garbage incineration facilities or the like is disposed of into the atmosphere without being converted into other energy forms. In this way, we human beings are wasting a tremendous amount of thermal energy, with a very limited amount being acquired through processes such as burning fossil fuels.
One effective means of increasing the proportion of available energy is to utilize the thermal energy that is being released into the atmosphere. This can be effectively accomplished by thermoelectric conversion, by which thermal energy is directly converted into electrical energy. Thermoelectric conversion is a method for converting energy in which, utilizing the Seebeck effect, electric power is generated by giving a temperature difference between both ends of a thermoelectric material to produce an electric potential difference. More specifically, in this thermoelectric power generation, electric power is generated simply by setting one end of a thermoelectric material in a region that has been heated to a high temperature by waste heat, setting the other end of the material in the atmosphere (ambient temperature), and then connecting a conductor to each of these ends. This method completely eliminates any need for movable devices, such as motors or turbines, which are usually required for generating electric power, thus leading to cost reduction, and eliminating the emission of gases produced by combustion, etc. The method also allows electric power to be generated continuously until the thermoelectric material is deteriorated.
Due to the advantages described above, thermoelectric power generation has been recognized as a technique that will help to resolve the energy-related problems that are likely to arise in the future. To perform thermoelectric power generation, it is necessary to develop thermoelectric materials having high thermoelectric conversion efficiency, as well as excellent heat resistance and chemical durability, etc. Substances presently known to have high thermoelectric conversion efficiency are intermetallic compounds. Particularly, TeAgSb-containing metallic compounds have high conversion efficiency at temperatures ranging from about 600 to 1,000K, i.e., the temperature region of waste heat. However, Te and Sb are rare and toxic elements, and cannot be used in air because they are readily oxidizable. Such drawbacks limit the extent to which TeAgSb-containing metallic compounds can be used as practical materials. It is thus hoped to develop materials that are composed of elements which are abundantly available but of low toxicity, and which have superior heat resistance and chemical durability, as well as high thermoelectric conversion efficiency.
While metallic oxides are conceivable as materials having excellent heat resistance and chemical durability, their thermoelectric conversion efficiency is lower by an order of magnitude compared with that of TeAgSb-containing metallic compounds. In fact, known oxides having high electric conductivity, i.e., those whose electrical resistivity is about 10 mΩ cm or less, show a Seebeck coefficient as low as a few tens of μV/K or less.
To address this problem, various studies have been conducted to search for oxides that have high thermoelectric conversion efficiency. For instance, Japanese Patent Nos. 3089301 and 3069701 disclose some composite oxides as such oxides.
In order to make practical use of composite oxides as thermoelectric materials, it is hoped that a method will be developed by which composite oxides with even higher thermoelectric conversion performance can be produced using a simple process.